Summiatry. Cotton seedllings were stubjected to a 96 hour chilling treatment (50 or 10°) after periods of germination at 310 ranging from 0 to 48 hours. Inhibition of stubsequeint growth at a favorable temperature by chilling was dependent on level of low temperature and stage of seedling development when chilled. Two periods of chilling hypersensitivitv were observed during germination: 1) coincident with subjection of seed to a germination environmenit; and 2) after 18 to 30 hours of germination at 310. Subsequent growth of seedlings chilled after 12 to 18 hours or 48 hoturs of germination at 310 was relatively unaffected. It is suggested that chilling alters specifically timed e-ents that occur at the initiation of germination and after 18 to 30 hoturs of germiniation, and that alteration of these germination processes is visited oIn long term subsequent growth of the plant. Materials and MethodsThe general experimental procedtu res involved application of low temperature treatnlenits to cotton seedlings at variouts times after germinationi was initiated, and then transference of the seedlings to liquid nutitrient culture in the greenhouse for evaluation of possible effects on stubsequent growth.The seed of Gossypium hirsu turnl L. utsed in all the investigations were from a single lot of selfpollinated M-8 genetic strain, which is a colchicine dotibled-haploid derived from Delta Pine 14 cutltivar. The seed coats were removed to eliminate -variation in germiinationi dute to seed-coat-induiced differential water tuptake. The seed was germinated between 2 rolled 31 X 46 cm germinatioin papers wet with 60 ml of a 0.5 strength modified Hoag--land's (1) nutrient soluition ancl covered with a waxed paper covering. Each seed roll contained 25 see (ds. Chilling consisted of temperatuire treatments at either 50 or 100 for a period of 96 hours. Treatments were applied to seeds in rolls after each of the following periods of germination at 310°0, 12, 18, 24, 30, 36, 42, and 48 houirs. Each test incltided treatments at the 8 stages of germiniationi and a control consisting of seedlings germinatedl 48 hours at 310. Cold treatecl seedllings likewise received a total of 48 hoturs at 310 in additioin to the 96 houirs of coldl. Treatments were schedulled so that all wxere completed simuiltaneouisly. Excess seedlings were incltided in each treatment so that only visuially inormal seedlings were transferred to the greenhouise anid cuilttured in an aerated liqutid nutrient soluitioni. Seedlings with aborted root tips, cortical collapse, or other abinormalities were discarded.The tests were arrangedl in a randomized block design wvith 5 replications. The plants were harvested after 15 days, (livided into tops aind root 431 www.plantphysiol.org on September 2, 2018 -Published by Downloaded from
Abstract. Cottonseed exposed to a temperature of 50 during hydration are killed, or the subsequent germination performance of surviving seed is seriously inhibited. Exposure to chilling for periods as short as 30 minutes reduces germination speed and induces root abnormalities, while chilling for 12 hours may kill all seed. Sens.itivity to chilling persists during the initial 2 to 4 hours of hydration. Seeds imbibed 4 hours at 31°, then dried, retain immunity to chilling. An irreversible event that is blocked or disrupted by chilling apparently occurs during early seed hydration.Chi-lling below 100 during initial water uptake of germinating cottonseed causes raidide abortion, followed by basal hyipocotyl swelling and, at times, Funigi are credited wiith inciting root tip injury in seedling cot-ton (10). Chemical agents are al,so known to influence radicle abnormalfities. Napthalene acetic acid causes a nub root !symptom when aipplied aSt concentrations of 1 mg/Al or greater (4); and nitrate in excess of 300 mg/il in the soil solution likewise induces radicle injury (9). The only reported beneficiall chemical application iis a calcium sul.fate seed treatment, which reduces the incidence of radicle afbortion in partially deteriorated 'seed (414).The presenit paper provides additional informati'on on: (1) the temperature-time relationsfhips necessary to induce radicle abortion, and (2) some preconditioning treatments for greatly reducing sensitivity of imbibing cottonseed to chilling. Materials and Methods'TIhree types of experiments were coiiducted to answer the following questions: A) how much time at low temperature during iiibi'bition is necessary to induce injury; B) what iis the duration of the colid sensitive period during iniltial seed hydration; and C) what preconditioning treatments can be applied to seed to reduce sensitivity to chilling during imbibition?The seerd used in ajl,l experiments were selfpollinated Gossypium hirsutum11 L. M-8 genetic selection, a douible-haploid derived fr-om Deltapine 14 cultivar. The seeds were produced, harvested, and stored 'under optimuim conditions to maintain high qual-ity and vigor. The germination procedure was as follows: 30 seeds were placed between 2 sheets olf 12 X 18 inch germination pape-r, m-oiistened with 70 ml of distililed water, and rolled with a waxed paper outer cover. All treatments were in 4 replications. In treatments requiring loiw temperature at the beginning of seed hydration the seeid rotlls were moistened with water which was at the assigned low tempperalture.To determine the amount of itime alt low temperature during seed hydtration required to induice *subsequent growth reta-ardation or visible symptoms of injury, seeds in germina!tion rol,l's were subjected to 50 at the beginnin(g of hydration for a series of time intervail trealtments (0.5, 1, 2, 4, 8, 16, or 24 hours) beefore germination for 3 days at a favorable temperature (310). Seedling development was measured by determining the ra!tio (3) of tihe roothypocotyl dry weight to total seed...
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